CN114345319B - A modified nano-cerium dioxide catalyst, preparation method thereof and application thereof in the synthesis of dimethyl carbonate - Google Patents

Abstract

The invention relates to the technical field of chemical industry, in particular to a modified nano cerium oxide catalyst and a preparation method thereof, and application of the catalyst in direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The method is a freeze drying method in the process of preparing the nano cerium oxide catalyst by a hydrothermal method, and can effectively improve the catalytic activity of the nano cerium oxide catalyst on the methyl esterification reaction of carbon dioxide. The preparation method of the modified nano cerium dioxide can obtain a high-activity catalyst, and when the catalyst is used for the reaction of synthesizing dimethyl carbonate from carbon dioxide and methanol, a dimethyl carbonate product can be obtained with high yield, and the selectivity of the dimethyl carbonate is close to 100%.

Description

A modified nano-cerium dioxide catalyst, its preparation method and its application in the synthesis of dimethyl carbonate

Technical Field

The invention relates to the technical field of chemical engineering, and in particular to a modified nanometer cerium dioxide catalyst and a preparation method and application thereof, and in particular to application of the catalyst in direct synthesis of dimethyl carbonate from carbon dioxide and methanol.

Background technique

Dimethyl carbonate (DMC) is considered to be an environmentally friendly green chemical because it contains methoxy and carbonyl groups in its molecules. Therefore, it can replace highly toxic dimethyl sulfate and monohalomethane as methylating agents, and replace phosgene as carbonylating agents. In addition, DMC is often used to replace environmentally harmful methyl tert-butyl ether as a gasoline additive and octane improver due to its high oxygen content. There are generally three traditional methods for the synthesis of DMC: phosgene method, methanol oxidative carbonylation method and ester exchange method. The direct synthesis of DMC from _CO2 and methanol can effectively solve the problems of highly toxic and corrosive in traditional methods. However, the high activation energy barrier of _CO2 and the thermodynamic limitations of the reaction make finding a suitable catalyst the key to the reaction.

In recent years, as the concept of green chemistry has gained popularity, the search for efficient catalysts for the synthesis of DMC from _CO2 and methanol has attracted widespread attention. Nano cerium dioxide and metal-doped cerium-based catalysts are currently hot topics of research. The patent document with publication number CN104841414A discloses a nano cerium dioxide catalyst, which introduces temperature disturbance factors during the ordinary hydrothermal preparation process, and changes the catalytic activity of the catalyst by changing the number of temperature disturbances. The DMC yield reaches 16.78mmol g ^-1 cat. The patent document with publication number CN112823879A discloses a metal-doped cerium-based catalyst, which has a high specific surface area, low pressure drop and excellent stability. The conversion rate of methanol at 140°C is 24.3%, and the selectivity of dimethyl carbonate is 78.6%. Patent document No. CN110479236A discloses a catalyst composed of multiple oxides, which is prepared from one to three of _{CeO2 , La2O3} and _ZrO2 in different mass ratios, with a DMC yield of 4.13 mmol g ^-1 cat and a selectivity of 100%. However, the catalytic performance of these catalysts is far from reaching the level of industrial application.

Therefore, on the basis of existing technology, the preparation of efficient, highly selective and highly stable _CeO2 catalysts through the improvement of catalyst preparation methods is a key step in realizing the industrialization of direct synthesis of DMC from _CO2 and methanol.

Summary of the invention

The invention provides a modified nanometer cerium dioxide catalyst, a preparation method thereof and application thereof in the synthesis of dimethyl carbonate. The modified nanometer cerium dioxide catalyst has better catalytic performance when applied to the synthesis of dimethyl carbonate and can effectively improve the conversion rate of the reactant methanol.

In order to solve the above technical problems, the technical solution of the present invention is as follows:

A method for preparing a modified nano-cerium dioxide catalyst comprises the following steps:

(1) adding a cerium nitrate solution dropwise into a sodium hydroxide solution to obtain a cerium hydroxide suspension under stirring;

(2) adding the cerium hydroxide suspension obtained in step (1) into a reaction kettle for aging, then washing the cerium hydroxide with alcohol and water, dispersing the obtained cerium hydroxide in deionized water, ultrasonically shaking, freezing overnight, and then freeze-drying;

(3) calcining the dried cerium hydroxide powder to obtain the nano cerium dioxide catalyst.

The above method uses a freeze-drying method in the process of preparing nano-cerium dioxide by a hydrothermal method to increase the specific surface area, porosity and oxygen vacancy content of the catalyst, thereby improving the activity of the catalyst.

In step (1), 30 to 50 mL of deionized water is used to dissolve 8 to 10 g of sodium hydroxide, and 5 to 10 mL of deionized water is used to dissolve 0.8 to 1 g of cerium nitrate.

Preferably, in step (1), the stirring speed is 400 to 800 rpm, and the stirring time is 20 to 40 min;

In step (2), the aging time is 10 to 14 hours, the alcohol washing and water washing times are both 3 to 5 times, and the ultrasonic time is 20 to 40 minutes.

Preferably, in step (2), the freeze-drying temperature is -60 to 80°C, and the freeze-drying time is 10 to 14 hours.

Preferably, in step (3), the calcination temperature is 600-800°C, the calcination time is 4-6h, and the heating rate is 2-5°C/min.

The present invention also provides a modified nano-cerium dioxide catalyst, which is prepared by the preparation method;

The modified nano-cerium dioxide catalyst has a short rod-shaped nano-cerium dioxide.

The present invention also provides a method for preparing dimethyl carbonate. Under the action of the modified nano-cerium dioxide catalyst and the dehydrating agent 2-cyanopyridine, carbon dioxide and methanol undergo a synthesis reaction to obtain the dimethyl carbonate.

The presence of a large number of oxygen vacancies and acidic and alkaline sites on the surface of nano-cerium dioxide catalysts promotes the adsorption and activation of carbon dioxide, showing good catalytic activity and selectivity. In order to improve the yield of the reaction, adding a dehydrating agent to remove the H ₂ O generated in the reaction is a very effective method. A large number of dehydrating agents for the direct synthesis of DMC from CO ₂ and methanol have been reported. 2-cyanopyridine, as a chemical dehydrating agent, is a more effective dehydration method that can effectively improve the reaction efficiency.

Preferably, the mass ratio of the modified nano-cerium dioxide catalyst to methanol is 1:50-100, the _CO2 pressure is 3-5 MPa, and the mass ratio of methanol to 2-cyanopyridine is 1:1-1.5.

Preferably, the temperature of the synthesis reaction is 120-160° C., the reaction time is 1-5 h, and the stirring speed is 400-800 rpm.

Preferably, the dehydrating agent 2-cyanopyridine forms a water-absorbing product 2-pyridineamide after the synthesis reaction is completed, and the water-absorbing product 2-pyridineamide is dehydrated to generate 2-cyanopyridine as a liquid raw material and returned to the reactor.

Compared with the prior art, the beneficial effects of the present invention are as follows:

The catalyst prepared by the present invention uses freeze drying as a drying method, so that the catalyst has good catalytic activity, especially in the process of synthesizing dimethyl carbonate from methanol and carbon dioxide, a significant catalytic synthesis effect is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a TEM image and particle size distribution of the freeze-dried nano-cerium dioxide catalyst prepared by the present method.

Detailed ways

The present invention is further described below in conjunction with examples. This example is implemented based on the technical solution of the present invention, and provides detailed implementation methods and processes, but the protection scope of the present invention includes but is not limited to the following examples.

Example 1

Dissolve 9.6g of sodium hydroxide powder in 35mL of deionized water, stir (600rpm), dissolve 0.86g of cerium nitrate in 5mL of water, ultrasonically shake, wait until the cerium nitrate is completely dissolved, add it dropwise into the sodium hydroxide solution, and stir for 30min. Add the suspension to a 50mL polytetrafluoroethylene liner and age it at 90°C for 12h. Then disperse it with anhydrous ethanol, centrifuge it for three minutes (8000rpm), repeat it three times, then add water to disperse it, centrifuge it for three minutes (8000rpm), repeat it three times. Disperse the cleaned cerium hydroxide in 5ml of deionized water, ultrasonically shake it, and then freeze it in a refrigerator overnight. Then freeze-dry it at a drying temperature of -70°C for 12h. Calcine the dried cerium hydroxide powder at 600°C for 5h to obtain a nano cerium dioxide catalyst.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 3 h, the reaction temperature was 140° C., the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 8 g of 2-cyanopyridine.

Comparative Example 1

Dissolve 9.6g of sodium hydroxide powder in 35mL of deionized water and stir (600rpm), dissolve 0.86g of cerium nitrate in 5mL of water, ultrasonically shake, wait until the cerium nitrate is completely dissolved, add it dropwise into the sodium hydroxide solution, and stir for 30min. Add the suspension to a 50ml polytetrafluoroethylene liner and age it at 90℃ for 12h. Then disperse it with anhydrous ethanol, centrifuge it for three minutes (8000rpm), repeat it three times, then add water to disperse it, centrifuge it for three minutes (8000rpm), repeat it three times. Then vacuum dry it at 70℃ for 12h. Calcine the dried cerium hydroxide powder at 600℃ for 5h to obtain a nano cerium dioxide catalyst.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 3 h, the reaction temperature was 140° C., the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 8 g of 2-cyanopyridine.

Comparative Example 2

Dissolve 9.6g of sodium hydroxide powder in 35mL of deionized water and stir (600rpm), dissolve 0.86g of cerium nitrate in 5mL of water, ultrasonically shake, wait until the cerium nitrate is completely dissolved, add it dropwise into the sodium hydroxide solution, and stir for 30min. Add the suspension to a 50mL polytetrafluoroethylene liner and age it at 90℃ for 12h. Then disperse it with anhydrous ethanol, centrifuge it for three minutes (8000rpm), repeat it three times, then add water to disperse it, centrifuge it for three minutes (8000rpm), repeat it three times. Then blow dry it at 70℃ for 12h. Calcine the dried cerium hydroxide powder at 600℃ for 5h to obtain a nano cerium dioxide catalyst.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 3 h, the reaction temperature was 140° C., the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 8 g of 2-cyanopyridine.

Table 1 shows the methanol conversion rate and DMC yield of the nano-cerium dioxide catalysts prepared under different drying conditions in Example 1 and Comparative Examples 1-2 for synthesizing dimethyl carbonate.

Comparative Example 3

The catalyst was prepared in the same manner as in Example 1.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 3 h, the reaction temperature was 140° C., the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 8 g of 3-cyanopyridine.

Example 2

The catalyst was prepared in the same manner as in Example 1.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 3 h, the reaction temperature was 140° C., the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 12.5 g of 2-cyanopyridine.

Example 3

The catalyst was prepared in the same manner as in Example 1.

The prepared nano-cerium dioxide catalyst was used for direct synthesis of dimethyl carbonate from carbon dioxide and methanol. The reaction pressure was 3.5 MPa, the reaction time was 4 h, the reaction temperature was 140°C, the stirring speed was 500 rpm, the catalyst dosage was 0.1 g, the methanol dosage was 11 g, and the dehydrating agent was 12.5 g of 2-cyanopyridine.

Table 1 Comparison of catalytic performance

The liquid product was qualitatively and quantitatively analyzed using a gas chromatograph (FULI 9790Ⅱ) equipped with a hydrogen flame ionization detector (FID) and a SE-54 capillary column (50m×0.32mm×1um). The results showed that when the freeze-drying method was used, the catalyst activity was significantly improved, the DMC yield reached 873mmol g ^-1 cat, and the selectivity was close to 100%, which far exceeded the previously reported results.

Claims (6)

1. A method for preparing dimethyl carbonate, characterized in that, under the action of a modified nano-cerium dioxide catalyst and a dehydrating agent 2-cyanopyridine, carbon dioxide and methanol undergo a synthesis reaction to obtain the dimethyl carbonate; The preparation method of modified nano-cerium dioxide catalyst comprises the following steps: (1) adding a cerium nitrate solution dropwise into a sodium hydroxide solution to obtain a cerium hydroxide suspension under stirring; (2) adding the cerium hydroxide suspension obtained in step (1) into a reaction kettle for aging, then washing the cerium hydroxide with alcohol and water, dispersing the obtained cerium hydroxide in deionized water, ultrasonically shaking, freezing overnight, and then freeze-drying; (3) calcining the dried cerium hydroxide powder to obtain the nano cerium dioxide catalyst; In step (2), the freeze-drying temperature is -60 to 80° C., and the freeze-drying time is 10 to 14 hours; The modified nano-cerium dioxide catalyst has a short rod-shaped nano-cerium dioxide. 2. The method for preparing dimethyl carbonate according to claim 1, characterized in that in step (1), the stirring speed is 400-800 rpm and the stirring time is 20-40 min; In step (2), the aging time is 10 to 14 hours, the alcohol washing and water washing times are both 3 to 5 times, and the ultrasonic time is 20 to 40 minutes. 3. The method for preparing dimethyl carbonate according to claim 1, characterized in that in step (3), the calcination temperature is 600-800° C., the calcination time is 4-6 h, and the heating rate is 2-5° C./min. 4. The method for preparing dimethyl carbonate according to any one of claims 1 to 3, characterized in that the mass ratio of the modified nano-cerium dioxide catalyst to methanol is 1:50-100, the _CO2 pressure is 3-5 MPa, and the mass ratio of methanol to 2-cyanopyridine is 1:1-1.5. 5. The method for preparing dimethyl carbonate according to any one of claims 1 to 3, characterized in that the temperature of the synthesis reaction is 120 to 160°C, the reaction time is 1 to 5 hours, and the stirring speed is 400 to 800 rpm. 6. The method for preparing dimethyl carbonate according to any one of claims 1 to 3, characterized in that the dehydrating agent 2-cyanopyridine forms a water-absorbing product 2-pyridineamide after the synthesis reaction is completed, and the water-absorbing product 2-pyridineamide is dehydrated to generate 2-cyanopyridine as a liquid raw material and returned to the reactor.